Exhaust gas purification system for internal combustion engine

ABSTRACT

An exhaust gas purification system includes a feedforward controller  47 , a feedback controller  49  outputting a correcting variable for achieving a target temperature of DPF  7 , and a variable adding unit  51  that adds the correcting variable output from the feedback controller  49  to a basic variable output from the feedforward controller  47  to compute a manipulated variable. The system further includes either one of an integrator resetter  55  that resets the integral value of an integrator that forms the feedback controller  49  when the amount of exhaust gas has suddenly dropped, and a basic variable calculating unit that calculates the basic variable to be output from the feedforward controller based on a signal representing the flow rate of exhaust gas.

TECHNICAL FIELD

The present invention relates to a diesel engine exhaust gaspurification system, and more particularly to the control of temperatureat the inlet of a diesel particulate filter (hereinafter, DPF) thatcollects particulate matter (hereinafter, PM) contained in the exhaustgas, during regeneration of the DPF.

BACKGROUND ART

PM reduction is as important as NO_(X) reduction in exhaust gasregulations of diesel engines, DPF is known as an effective technique inthis regard.

DPF is a PM collecting device that uses a filter. As the PM continues toaccumulate in the DPF in engine operating conditions with low exhaustgas temperatures, forced regeneration is carried out wherein thetemperature is forcibly raised to burn the PM.

Common means of raising the temperature include delaying the fuelinjection timing, post-injection, and intake throttling, which allencompass the problem of adversely affecting the fuel economy. On theother hand, higher temperatures mean a quick and efficient forcedregeneration of DPF with a smaller decrease of fuel economy since thehigher the temperature, the higher the speed of burning the PM.

However, if the DPF temperature is too high, the PM burn rapidly and theDPF temperature rises quickly, which may damage the DPF or deterioratethe catalyst carried in the DPF.

Temperature control is therefore necessary, to maintain the DPFtemperature at a level suitable for the regeneration, so as to prevent adrop in the fuel economy and ensure safe regeneration of the DPF.

There is Japanese Patent Application Laid-open No. 2005-320962 (PatentDocument 1) as an example of temperature raising control in forcedregeneration of DPF. The Patent Document 1 describes a process oftemperature control during regeneration of DPF wherein an optimalfeedback gain in accordance with the operating condition is used toachieve both of stability and responsivity of temperature feedbackcontrol to raise the temperature to a target level.

There is a time delay between variable manipulation to raise thetemperature and a change in the exhaust gas temperature. The time delayof control targets also varies depending on the changes in the operatingcondition. For example, an increase in the exhaust gas flow rateincreases the heat transfer coefficient and decreases the time delay,while a decrease in the exhaust gas flow rate increases the time delaybetween changes in variable and changes in exhaust gas temperature aswell as the time constant, whereby the time delay is increased.

Described in the document is a corrective action performed inconsideration of the time delay to make the temperature closer to thetarget level quickly, wherein the operating condition is detected todetermine the current time delay from the relationship between theexhaust gas flow rate and the time delay that is known from theoperating condition, an optimal feedback gain is calculated inaccordance therewith, and the temperature raising variables arecorrected using this feedback gain.

Patent Document 1<Japanese Patent Application Laid-open No. 2005-320962

Patent Document 1 describes making the temperature closer to a targetlevel quickly by correcting a feedback gain as described above. However,it is not possible to improve the stability of control of the DPF inlettemperature by a corrective action using only a feedback gain,particularly when the exhaust gas flow rate has decreased, since, insuch a condition, the time delay between changes in variable(post-injection amount) and changes in exhaust gas temperature isincreased, and so is the time constant, because of which the exhaust gastemperature control performance is deteriorated, i.e., it takes longuntil a change appears in the DPF inlet temperature even when, forexample, the post-injection amount is excessive.

If the temperature is controlled properly by feedforward control atvarious operating condition points, the feedback variables will be zeroin a steady state and the problem associated with the feedback controldescribed above will not arise. In a small general-purpose engine,however, in which the rpm and the load change independently in use, itis difficult to set the feedforward variables properly in all operatingconditions.

If, for example, the flow rate of the exhaust gas has dropped largely ina short time, and the flow rate remains low after that, the DPF inlettemperature will rise, and when this phenomenon appears, it is difficultto solve it by improvement of the gain in the feedback control, or byoptimization of controlled variables in the feedforward control.

DISCLOSURE OF THE INVENTION

Accordingly, the present invention was made in view of these problems,and it is an object of the invention to provide an exhaust gaspurification system for an internal combustion engine capable of stablecontrol to keep the DPF inlet temperature at a target level even whenthe flow rate of exhaust gas remains low after a sudden drop in the flowrate.

To solve the problems described above, the present invention provides anexhaust gas purification system for an internal combustion engine thatincludes a diesel oxide catalyst (DOC) and a diesel particulate filter(DPF) for collecting particulate matter (PM) in exhaust gas in anexhaust gas passage and that treats the PM collected in the DPF toregenerate the DPF, the system including: a regeneration control unitcontrolling a temperature raising unit, when the PM has accumulated morethan a predetermined amount, to heat up the DPF to around apredetermined target temperature to burn off the accumulated PM, theregeneration control unit including a feedforward controller outputtinga basic variable for the temperature raising unit based on an operatingcondition of the internal combustion engine, a feedback controlleroutputting a correcting variable for achieving the target temperature ofthe DPF, and a variable adding unit adding the correcting variableoutput from the feedback controller to the basic variable output fromthe feedforward controller to compute a manipulated variable. The systemfurther includes at least one of an integrator resetter resetting anintegral value of an integrator forming the feedback controller when asudden drop in exhaust gas flow rate is detected based on a monitoredexhaust gas flow rate or a control value calculated from the exhaust gasflow rate, and a basic variable calculating unit calculating the basicvariable to be output from the feedforward controller based on theexhaust gas flow rate or a control value calculated from the exhaust gasflow rate.

According to the invention, there is provided an integrator resetterthat resets the integral value of the integrator forming the feedbackcontroller when a sudden drop in the flow rate of exhaust gas isdetected based on the flow rate of exhaust gas passing through the DPTor a control value calculated from the exhaust gas flow rate, so thatthe late post-injection amount, which is the manipulated variable toraise the temperature, is prevented from being adversely affected by theintegral value remaining in the integrator in the PID controller. As aresult, the DPT inlet temperature can be kept at around the target leveleven when the flow rate of exhaust gas has dropped suddenly.

The feedforward controller outputs a basic variable for the temperatureraising unit based on the operating condition of the internal combustionengine. In an operating condition in which the flow rate of the exhaustgas has decreased suddenly and remains low for a while, in theconventional technique, when there was (accumulated) still an integralvalue before the sudden drop of the exhaust gas flow rate in theintegrator of the feedback controller, the response of the DPF inlettemperature was slow (deadtime was long) particularly when the flow ratewas decreasing, so that it would take time to output accumulatedintegral value, during which the integral values are added as correctingvariables, whereby the DPF inlet temperature was raised, resulting in aloss of the controllability.

In the present invention, as there is provided an integrator resetterthat resets the integral value of the integrator, such loss ofcontrollability of the DPF inlet temperature caused by a remainingintegral value is prevented.

Alternatively, there is provided a basic variable calculating unitcalculating a basic variable of the feedforward controller based on theflow rate of exhaust gas or a control value calculated from the exhaustgas flow rate. Namely, there is provided a basic variable calculatingunit calculating a basic variable by using an equation, morespecifically, a transfer function that models the temperature risingcharacteristics of the exhaust gas in the DOC, so that proper basicvariables can be obtained under various operating conditions.

Therefore, as compared to using a map prepared based on variousoperating conditions beforehand, the feedforward manipulated variablescan be properly determined under various operating conditions of a smallgeneral-purpose engine, in which the rpm and the load independentlychange in use, and thus the controllability of the DPF inlet temperaturecan be improved.

In the present invention, preferably, it may be determined that therehas been a sudden drop in the flow rate of exhaust gas when any of thefollowing applies:

(1) The rate of decrease of the exhaust gas flow rate is not higher thana threshold;

(2) The exhaust gas flow rate has decreased to a threshold or below; or

(3) The rate of decrease of the exhaust gas flow rate is not higher thana threshold as well as the exhaust gas flow rate has decreased to lessthan a threshold.

By monitoring the exhaust gas flow rate as well as the rate of decreasein the flow rate, the integrator value is prevented from being resetmore than necessary, despite frequent sudden drops in the exhaust gasflow rate during transient operation. This prevents a loss ofcontrollability of the DPF inlet temperature during transient operationwhen the engine rpm and the engine load continuously change.

Or,

(4) The flow rate of the exhaust gas remains not higher than thethreshold for a certain period of time or longer.

With the additional condition, wherein the flow rate of exhaust gasremains less than the threshold for more than a certain period of time,unnecessary resetting of the integral value during transient operationis prevented even more reliably.

In the present invention, preferably, the integral value of theintegrator of the PID controller forming the feedback controller may bereset when the integral value is positive.

By resetting the integral value only when it is positive, an unintendedincrease in the DPF inlet temperature caused by a resetting action canbe prevented. Namely, it is for preventing an unintended increase in theDPF inlet temperature, which would occur if the integral value of theintegrator of the feedback controller is reset when the integral valueis negative.

In the present invention, preferably, the basic variable calculatingunit may calculate the basic variable of the feedcontroller by using apreset equation of a transfer function modeling the temperature risingcharacteristics of the exhaust gas in the DOC in use of a deviation of ameasured DOC inlet temperature from the target DPF inlet temperature,and a control gain calculated based on the exhaust gas flow rate.

Namely, the temperature rising characteristics of the exhaust gas in theDOC are modeled by a primary transfer function, and a latepost-injection amount that can achieve a target DPF inlet temperature isobtained through calculation as the basic variable of the feedforwardcontroller.

More specifically, the late post-injection amount Z, or a basicvariable, is determined using a relational expression of the primarytransfer function Z=K/(1+σs)e, wherein e is the deviation of a measuredDOC inlet temperature from the target DPF inlet temperature, σ is thetime constant parameter and K is the control gain determined from theflow rate of exhaust gas.

The smaller the design parameter (adjusting parameter) σ is set, thehigher the sensitivity of the output will be relative to changes intemperature deviation e and K, and the larger σ is set, the lower theresponsivity.

The late post-injection amount, which is the basic variable, iscalculated using a control gain, which is a control value obtained fromthe exhaust gas flow rate, instead of setting proper basic variables bythe feedforward controller at various operating condition points, sothat, as compared to using a map prepared based on various operatingconditions beforehand, the feedforward variables can be properlydetermined under various operating conditions.

Since the late post-injection amount, which is the basic variable, isdetermined based on the deviation of the measured DOC inlet temperaturefrom the target value of the DPF inlet temperature, the integrator ofthe PID controller does not output a large value, i.e., a largedeviation from the target DPF inlet temperature is unlikely to occur, sothat a loss of the controllability of the DPF inlet temperature isprevented under an operating condition in which the flow rate of exhaustgas remains low after a drop in the exhaust gas flow rate in a shorttime.

In the present invention, preferably, the manipulated variable of thetemperature raising unit may represent an amount of late post-injectionthat is performed in a period after a main injection and does notdirectly contribute to combustion, after activation of the DOC.

The manipulated variable of the temperature raising unit shouldpreferably represent an amount of late post-injection that is performedin a period after a main injection and does not directly contribute tocombustion, after activation of the DOC.

The late post-injection in the present invention snail be describedhere.

Main injection is done to bring about main combustion in the combustionchamber. Early post-injection refers to an injection of fuel in asmaller amount than that of the main injection performed immediatelyafter the main injection when the pressure inside the cylinder is stillhigh. This early post-injection raises the temperature of the exhaustgas, and the hot exhaust gas flowing into the DOC activates the DOC. Asecond post-injection is performed after that, when the crank angle isnear the bottom dead center after the early post-injection. This secondpost-injection is called late post-injection, which does not contributeto the combustion inside the combustion chamber, so that the fuel isdischarged from the combustion chamber into the exhaust gas passage inthe exhaust stroke. This fuel discharged from the combustion chamberreacts in the already activated DOC, and the heat thus generated byoxidation further raises the exhaust gas temperature to a level of about600° C. necessary for the regeneration of the DPF, to promote burning ofthe PM.

According to the present invention, there is provided an integratorresetter that resets the integral value of the integrator forming thefeedback controller when a sudden drop in the flow rate of exhaust gasis detected based on the flow rate of exhaust gas passing through theDPF or a control value calculated from the exhaust gas flow rate, sothat the late post-injection amount, which is the manipulated variableto raise the temperature, is prevented from being adversely affected bythe integral value remaining in the integrator in the PID controller. Asa result, the DPF inlet temperature can be kept at around the targetlevel even when the flow rate of exhaust gas has dropped suddenly.

Alternatively, there is provided a basic variable calculating unitcalculating a basic variable of the feedforward controller based on theflow rate of exhaust gas passing through the DPF or a control valuecalculated from the exhaust gas flow rate, so that proper basicvariables can be obtained through calculation under various operatingconditions. Therefore, as compared to using a map prepared based onvarious operating conditions beforehand, the feedforward manipulatedvariables can be properly determined under various operating conditionsof a small general-purpose engine, in which the rpm and the loadindependently change in use, and thus the controllability of the DPFinlet temperature can be improved.

As the controllability of the DPF inlet temperature is improved, thetarget temperature at the DPF inlet can be set several tens of ° C.higher without the possibility of the DPF reaching the temperature atwhich the catalyst held therein is degraded. Thus thermal degradation ofthe catalyst held in the DPF is prevented, whereby the durability of theDPF is improved.

As the target temperature at the DPF inlet can be set higher, the timefor controlling the DPF regeneration is reduced, whereby the problem ofoil dilution caused by late post-injection at the time of regenerationis resolved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic configuration diagram of a diesel engine exhaustgas purification system according to one embodiment of the presentinvention;

FIG. 2 is a configuration block diagram showing a first embodiment of aregeneration control unit;

FIG. 3 is a configuration block diagram showing a second embodiment ofthe regeneration control unit;

FIG. 4 is a control flowchart of the first embodiment;

FIG. 5( a) is a subroutine flowchart showing one of the steps of theflowchart of FIG. 4 in detail;

FIG. 5( b) is a subroutine flowchart showing one of the steps of theflowchart of FIG. 4 in detail;

FIG. 5( c) is a subroutine flowchart showing one of the steps of theflowchart of FIG. 4 in detail;

FIG. 5( d) is a subroutine flowchart showing one of the steps of theflowchart of FIG. 4 in detail;

FIG. 6 is a control flowchart of the second embodiment;

FIG. 7( a) and FIG. 7( b) are subroutine flowcharts showing steps of theflowchart of FIG. 6 in detail;

FIG. 8 is a diagram for explaining the confirmation test results of thefirst embodiment; and

FIG. 9 is a characteristics chart for explaining a gain K map.

BEST MODE FOR CARRYING OUT THE INVENTION

The illustrated embodiments of the present invention will be hereinafterdescribed in detail. It should be noted that, unless otherwiseparticularly specified, the sizes, materials, shapes, and relativearrangement or the like of constituent components described in theseembodiments are not intended to limit the scope of this invention.

The overall configuration of the diesel engine exhaust gas purificationsystem according to the present invention will be described withreference to FIG. 1.

As shown in FIG. 1, an exhaust gas passage 3 of a diesel engine(hereinafter referred to as engine) includes an exhaust gas aftertreatment system 9 including a DOC (oxidation catalyst) 5 and a DPF(particulate filter) 7 that collects soot downstream of the DOC 5.

In the exhaust gas passage 3 is also disposed an exhaust gasturbocharger 13 having an exhaust gas turbine 11 a and a compressor 11 bcoaxially driven with the turbine. Intake air discharged from thecompressor 11 b of the exhaust gas turbocharger 13 flows through anintake air passage 15 and enters an intercooler 17 to be cooled. Anintake throttle valve 19 controls the amount of intake air flowingtherethrough, and after that, the air flows from an intake manifold 21into combustion chambers through intake ports of respective cylindersvia intake valves of the engine 1.

Although not shown, the engine 1 includes a common rail fuel injectionsystem that controls the timing of injection, and the amount andpressure of fuel to be injected into the combustion chambers. The commonrail fuel injection system feeds fuel at a predetermined controlledpressure to the fuel injection valves 22 of the respective cylinders ata predetermined timing of injection.

An exhaust gas recirculation (EGR) passage 25 bifurcates from midway ofthe exhaust gas passage 3 or an exhaust manifold 23 so as to introducepart of the exhaust gas to a portion downstream of the intake throttlevalve 19 through an EGR cooler 27 and an EGR valve 29.

Combustion gas or exhaust gas 31 produced in the combustion chambers ofthe engine 1 flows through the exhaust manifold 23 that connects to eachof the exhaust ports of the cylinders and through the exhaust passage 3,and spins the exhaust gas turbine 11 a of the exhaust gas turbocharger13 to drive the compressor 11 b, after which it flows into the exhaustgas after treatment system 9 through the exhaust gas passage

The DPF 7 is disposed downstream of the DOC 5. The regeneration controlunit 33 of the DPF 7 receives signal inputs from an air flowmeter 35that detects the amount of air flowing into the compressor 11 b, a DOCinlet temperature sensor 37, and a DPF inlet temperature sensor 39.

Signals from an engine rpm sensor 41 and an engine load sensor 43 arealso input to the regeneration control unit (ECU) 33.

This regeneration control unit 33 controls a temperature raising unitwhen the amount of PM accumulated in the DPF 7 exceeds a predeterminedlevel to raise the temperature at the inlet of the DPF 7 to around atarget level (of about 600° C.) to burn off the accumulated PM.

The outline of the control by the regeneration control unit 33 to burnoff the PM will be described first,

When conditions for starting forced regeneration are met, which isdetermined based on, for example, mileage, running time of the engine, atotal amount of fuel consumed, in the case with a vehicle, and when theforced regeneration is started, the DOC temperature raising control isexecuted to activate the DOC 5. This DOC temperature raising controlinvolves reducing the degree of opening of the intake throttle valve 19to reduce the amount of air flowing into the combustion chambers, so asto increase unburnt fuel in the exhaust gas. A first post-injection isthen performed, wherein a smaller amount of fuel than the main injectionis injected immediately after the main injection when the pressureinside the cylinders is still high, this early post-injection raisingthe exhaust gas temperature without affecting the engine output. Thehigh-temperature exhaust gas flows into and activates the DOC 5, and asthe unburnt fuel in the exhaust gas is oxidized by the activated DOC 5,the exhaust gas temperature is further raised by the heat generated fromthe oxidation.

When the DOC inlet temperature is determined to have reached andexceeded a predetermined level, a late post-injection is carried out tofurther raise the inlet temperature of the DPF 7. The latepost-injection refers to a second post-injection after the earlypost-injection mentioned above, wherein fuel is injected when the crankangle is near the bottom dead center. By this late post-injection, fuelflows out from the combustion chambers to the exhaust gas passage 3 whenthe exhaust valves are open. The discharged fuel reacts at the activatedDOC 5, so that the exhaust gas temperature is further raised by the heatgenerated from the oxidation to achieve a temperature necessary for theregeneration of the DPF 7, e.g. 600° C., to promote burning of the PM.

First Embodiment

Next, a first embodiment of the control of the amount of latepost-injection by the regeneration control unit 33 will be describedwith reference to the control configuration block diagram of FIG. 2,

The regeneration control unit 33 controls the amount of the latepost-injection (manipulated variable) to constantly keep the inlettemperature of the DPF 7 at the setpoint of about 600° C. and for thatpurpose includes: A feedforward controller 47 that outputs a commandindicative of a basic injection amount (basic variable) of latepost-injection based on a feedforward control map 45 that defines basicinjection amounts preset in accordance with the engine rpms and fuelinjection amounts (engine loads); a feedback controller 49 that outputsa command indicative of a correction amount of late post-injection(correcting variable) for achieving the target temperature of the DPF 7;and an injection amount (manipulated variable) adding unit 51 (see FIG.2) that computes an amount of injection by adding the correction amountoutput from the feedback controller 49 to the basic injection amountoutput from the feedforward controller 47.

In the first embodiment, an integrator resetter 55 is further providedfor resetting the integral value of an integrator 53 that forms part ofthe feedback controller 49.

The feedforward controller 47 computes a basic injection amount, whichis a feedforward control command 57, based on the feedforward controlmap 45 that defines basic amounts of injection preset in accordance withthe engine rpms and fuel injection amounts (engine loads) that representvarious operating conditions of the engine, as described above.

The feedback controller 49, on the other hand, includes a targettemperature setting unit 59 that sets an initial target value of the DPF7 inlet temperature at the start of the control and a target temperaturethereafter. The feedback controller inputs a target DPF 7 inlettemperature and the measured DPF 7 inlet temperature to anadder-subtracter 61, and performs feedback calculation at a PIDcontroller 63 using the deviation of the measured inlet temperature fromthe target inlet temperature obtained as an output signal of theadder-subtracter 61 to compute a correction amount of injection as afeedback control command 65.

The PID controller 63 calculates the proportional element (P) using aproportional gain Kp, the derivative element (D) using a derivative gainKd, and the integral element (I) using an integral gain Ki, and thecalculation results are all input to an adder 67 so as to compute thefeedback control command

The feedforward control command 57 and the feedback control command 65are input to the adder (injection amount adding unit) 51, which outputsan addition command 69. This addition command signal 69 is input to acommand saturation unit 71 to set a limit to the output signal forprotection of the DPF 7. The signal that has passed through the commandsaturation unit 71 is output as a late post-injection command signal.

Further, a PID auto-tuner 75 is provided for automatically tuning thefeedback controller 49. The auto-tuning is based on a deviation betweenthe output signal of the command saturation unit 71 and the outputsignal of the adder 51, which is provided from an adder/subtractor 73.The output signal from a calculation element 77 of the PID auto-tuner 75is input to an adder-subtractor 78 to be input to the integrator 53.

The PID auto-tuner 75 provided as an anti-windup measure (for preventinginput saturation) of the feedback controller 49 prevents the integralvalue of the integrator 53 of the PID controller 63 in the feedbackcontroller 49 from accumulating while the command saturation unit 71 islimiting the command. Thereby, the command following capability isimproved when the setpoint of the feedback control is changed.

In the first embodiment, there is further provided the integratorresetter 55 that resets the integral value of the integrator 53. Thisintegrator resetter 55 includes a sudden drop determination unit 79 thatdetermines whether or not the flow rate of exhaust gas has droppedsuddenly, so that the integral value of the integrator 53 is reset whenthis sudden drop determination unit 79 detects a sudden drop in theexhaust gas flow rate.

The control flow of this integrator resetter 55 will be described withreference to FIG. 4. Step S1 of FIG. 4 is shown in detail in FIG. 5( a),step S2 is shown in detail in FIG. 5( b), step S3 is shown in detail inFIG. 5( c), and step S4 is shown in detail in FIG. 5( d).

First, the rate of change in the flow rate of exhaust gas is determinedat step S1 in FIG. 4. This is done, as shown in FIG. 5( a), by firstcalculating the flow rate G_(ex) of exhaust gas at step S11. The exhaustgas flow rate is calculated based on a signal indicative of the flowrate of air G_(a) from the air flowmeter 35, and a signal indicative ofthe fuel injection command G_(f) from the common rail fuel injectionsystem (not shown), by the equation G_(ex)=G_(a)+G_(f). Next, the timederivative dG_(ex)/dt of the exhaust gas flow rate G_(ex) is calculatedat step S12. Step S13 determines whether or not the time derivativedG_(ex)/dt is less than a threshold K1, and if yes, Flag 1 is turned onat step S14, and if not, Flag 1 is turned off at step S15, after whichthe process is returned.

Referring back to the flow of FIG. 4, the flow rate of exhaust gas isdetermined at step S2. As shown in FIG. 5( b), step S21 determineswhether or not the exhaust gas flow rate G_(ex) is less than a thresholdK2, and if yes, Flag 2 is turned on at step S22, and if not, Flag 1 isturned off at step S23, after which the process is returned.

Next, the timer counts up at step S3 in the flow of FIG. 4. As shown inFIG. 5( c), step S31 determines whether or not Flag 1 is ON or Flag 3 isON, and if yes, step S32 determines whether or not Flag 2 is ON. If “No”at step S31, Flag 3 is turned off at step S35, and the timer is set tozero at step S36.

If step S32 determines that Flag 2 is ON, Flag 3 is turned on at stepS33, and Δt (cyclic time for processing the subroutine) is added to thetimer at step S34.

Next, the integral value is reset at step S4 in the flow of FIG. 4. Asshown in FIG. 5( d), step S41 determines whether or not the timer countexceeds a threshold K3, and if not, the process is returned, and if yes,step S42 determines whether or not the integral value exceeds athreshold K4. This is done by setting K4=0, for example, if the integralvalue is positive.

If the integral value is positive, the process goes to step S43, wherethe integral value is reset to zero. The timer is reset at step S44 andthe process is returned.

As described above, the sudden drop determination unit 79 determinesthat there has been a sudden drop in the flow rate of exhaust gas bydetermining the rate of change in the flow rate of exhaust gas at stepS1 and further by determining the flow rate of exhaust gas at step S2,so that, despite frequent sudden drops in the exhaust gas flow rateduring transient operation, the integral value is prevented from beingreset more than necessary by thus monitoring the exhaust gas flow rateas well as the rate of decrease in the flow rate.

This prevents a loss of controllability of the DPF inlet temperatureduring transient operation when the engine rpm and the engine loadcontinuously change

Furthermore, it is determined at step S3 that there has been a suddendrop in the exhaust gas flow rate when the condition in which theexhaust gas flow rate is not higher than the threshold has continued formore than a certain period of time. Namely, with the additionalcondition for the determination, wherein the flow rate of exhaust gasremains not higher than the threshold for a certain period of time orlonger as determined at step S41, unnecessary resetting of the integralvalue during transient operation is prevented even more reliably.

FIG. 8 shows the confirmation test results. FIG. 8( a) shows the flowrate of exhaust gas, which was decreased stepwise from a steady state.FIG. 8( b) shows the changes in the late post-injection amount at thistime, while FIG. 8( c) shows the changes in the temperature of theexhaust gas at the DPF inlet. Dotted lines in FIGS. 8( b) and 8(c)represent a conventional technique in which the integrator resetter 55is not provided, while solid lines represent the present inventionhaving the integrator resetter 55.

FIG. 8( b) shows that the late post-injection amount is decreased with afaster response speed than the conventional technique. FIG. 8( c) showsthat the exhaust gas temperature at the DPF inlet is prevented fromovershooting.

As described above, the first embodiment includes the integratorresetter 55 that resets the integral value of the integrator 53 formingthe feedback controller 49 when the sudden drop determination unit 79determines that the flow rate of exhaust gas has dropped suddenly basedon the flow rate of exhaust gas passing through the DPF 7 or the rate ofdecrease in the exhaust gas flow rate (control value) that is the timederivative of the flow rate. Thereby, the late post-injection amount,which is the manipulated variable to raise the temperature, is preventedfrom being adversely affected by the integral value remaining in theintegrator 53 in the PID controller 63. As a result, the DPF inlettemperature can be reliably kept at around the target level even whenthe flow rate of exhaust gas has dropped rapidly.

As the controllability of the DPF inlet temperature is improved, thetarget temperature at the DPF inlet can be set several tens of ° C.higher without the possibility of the DPF 7 reaching the temperature atwhich the catalyst held therein is degraded. Thus thermal degradation ofthe catalyst held in the DPF is prevented, whereby the durability of theDPF 7 is improved.

As the target temperature at the DPF inlet can be set higher, the timefor controlling the DPF regeneration is reduced, whereby the problem ofoil dilution caused by late post-injection at the time of regenerationis resolved.

Second Embodiment

Next, a second embodiment of the control of the amount of latepost-injection by the regeneration control unit 33 will be describedwith reference to the control configuration block diagram of FIG. 3.

The feedforward controller 47 is the same as that of the firstembodiment and therefore will not be described again. The integratorresetter 55 of the first embodiment is not provided here. The secondembodiment is characteristic in that it has a different feedforwardcontroller than that (47) of the first embodiment.

As shown in FIG. 3, unlike the first embodiment, the feedforwardcontroller 81 of the second embodiment does not output a commandindicative of a basic injection amount (basic variable) of latepost-injection based on a map that defines feedforward commandsdetermined through tests and preset beforehand in accordance withvarious operating conditions. Instead, the feedforward controlleroutputs a command indicative of a basic injection amount that itcalculates based on actual measurements of the exhaust gas flow rate andthe inlet temperature of the DOC 5 using a preset DOC transfer functionmodel.

The feedforward controller 81 includes an adder-subtractor (deviationcalculating unit) 83 that calculates a deviation e of a measured DOCinlet temperature from the target DPF inlet temperature, a gaincalculating unit 85 that calculates a control gain (control value) Kbased on the flow rate of exhaust gas, and a late post-injection amountcalculating unit (basic variable calculating unit) 87 that calculatesthe basic variable using a preset DOC transfer function model.

More specifically, the late post-injection amount calculating unit 87calculates a late post-injection amount Z as a basic variable using adeviation e of a measured DOC inlet temperature from the target DPFinlet temperature, design parameter (adjusting parameter) σ, and acontrol gain K determined from the flow rate of exhaust gas, by arelational expression of the primary transfer function Z=K/(1+σs)e

The control flow of the feedforward controller 81 will be described withreference to FIG. 6.

First, the temperature deviation e is calculated at step S110. Morespecifically, as shown in FIG. 7( a), the DOC inlet temperatureT_(DOCIN) is obtained at step S111, and the target DPF inlet temperaturerT_(DPFIN) is obtained at step S112. The temperature deviation e iscalculated at step S113 by e=rT_(DPFIN)−T_(DOCIN).

The flow rate of exhaust gas G_(ex) is calculated at step S120 in theflow of FIG. 6. This flow rate G_(ex) is calculated similarly to stepS11 as has been described in the foregoing. More specifically, as shownin FIG. 7( b), the flow rate is calculated by G_(ex)=G_(a)+G_(f) at stepS123, based on the flow rate of air G_(a) obtained from the airflowmeter 35 at step S121, and the fuel injection command G_(f) obtainedfrom the common rail fuel injection system (not shown) at step S122.

Next, at step S130 in the flow of FIG. 6, the exhaust gas flow rateinput is filtered using a low path filter 89 and by a primary delayprocess to remove noise. After that, at step S140, the control gain K isdetermined. The control gain K is obtained from a gain K map 91 such asthe one shown in FIG. 9, whereby a gain K is given relative to a certainexhaust gas flow rate. This gain K map 91 is determined in advance basedon test data or through simulation and calculation.

Step S150 computes a transfer function. The late post-injection amountZ, which is the basic variable, is determined using a relationalexpression of the primary transfer function Z=K/(1+σs)e, wherein e isthe deviation of a measured DOC inlet temperature from the target DPFinlet temperature, σ is the design parameter (adjusting parameter), andK is the control gain determined from the flow rate of exhaust gas. Thesmaller σ is set, the higher the sensitivity of the output will herelative to changes in temperature deviation e and K, and the larger σis set, the lower the responsivity.

After that, at step S160, the unit of the injection amount calculated atstep S150 is converted to compute a command value, and the process isreturned.

In this way, the late post-injection amount, which is the basicvariable, is calculated using a control gain, which is a control valuedetermined from the exhaust gas flow rate, instead of calculating abasic injection amount as a feedforward control command 57 using afeedforward control map 45 that defines proper basic variables atvarious operating condition points, as with the feedforward controller47 of the first embodiment. Therefore, as compared to using a mapprepared based on various operating conditions beforehand, thefeedforward variables can be properly determined under various operatingconditions of a small general-purpose engine, in which the rpm and theload independently change in use, and thus the controllability of theDPF inlet temperature can be improved.

Since the late post-injection amount, which is the basic variable, isdetermined based on the deviation of the measured DOC inlet temperaturefrom the target value of the DPF inlet temperature, the integrator 53 ofthe PID controller 63 does not output a large value, i.e., a largedeviation from the target DPF inlet temperature is unlikely to occur, sothat a loss of the controllability the DPF inlet temperature isprevented under an operating condition in which the flow rate of exhaustgas remains low after a drop in the exhaust gas flow rate in a shorttime.

It goes without saying that the configurations of the first and secondembodiments can be combined. In this case, the feedforward controller 81may be configured as in the second embodiment, and the integrator 53 ofthe PID controller 63 in the feedback controller 49 may include theintegrator resetter 55. With such a configuration, the controllabilityof the DPF inlet temperature can be improved even more,

INDUSTRIAL APPLICABILITY

The present invention enables stable control to keep the DPF inlettemperature at a target level even when the flow rate of exhaust gasremains low for a while after a drop in the flow rate, and therefore canbe suitably applied to a diesel engine exhaust gas purification system.

1. An exhaust gas purification system for an internal combustion enginethat includes a diesel oxide catalyst (DOC) and a diesel particulatefilter (DPF) for collecting particulate matter (PM) in exhaust gas in anexhaust gas passage and that treats the PM collected in the DPF toregenerate the DPF, the system comprising: a regeneration control unitcontrolling a temperature raising unit, when the PM has accumulated morethan a predetermined amount, to heat up the DPF to around apredetermined target temperature and burn off the accumulated PM, theregeneration control unit including a feedforward controller outputtinga basic variable for the temperature raising unit based on an operatingcondition of the internal combustion engine, a feedback controlleroutputting a correcting variable for achieving the target temperature ofthe DPF, and a variable adding unit adding the correcting variableoutput from the feedback controller to the basic variable output fromthe feedforward controller to compute a manipulated variable, the systemfurther comprising: at least one of an integrator resetter resetting anintegral value of an integrator forming the feedback controller when asudden drop in exhaust gas flow rate is detected based on a monitoredexhaust gas flow rate or a control value calculated from the exhaust gasflow rate, and a basic variable calculating unit calculating the basicvariable to be output from the feedforward controller based on theexhaust gas flow rate or a control value calculated from the exhaust gasflow rate.
 2. The exhaust gas purification system for an internalcombustion engine according to claim 1, wherein the integrator resetterdetermines that there has been a sudden drop in the exhaust gas flowrate when the rate of change of exhaust gas flow rate shows a decreaserate of less than a threshold.
 3. The exhaust gas purification systemfor an internal combustion engine according to claim 1, wherein theintegrator resetter determines that there has been a sudden drop in theexhaust gas flow rate when the exhaust gas flow rate has decreased toless than a threshold.
 4. The exhaust gas purification system for aninternal combustion engine according to claim 1, wherein the integratorresetter determines that there has been a sudden drop in the exhaust gasflow rate when the exhaust gas flow rate shows a decrease rate of lessthan a threshold, as well as when the exhaust gas flow rate hasdecreased to less than a threshold.
 5. The exhaust gas purificationsystem for an internal combustion engine according to claim 3, whereindetermination is made that there has been a sudden drop in the exhaustgas flow rate when the exhaust gas flow rate remains less than thethreshold for more than a certain period of time.
 6. The exhaust gaspurification system for an internal combustion engine according to claim1, wherein the integral value of the integrator forming the feedbackcontroller is reset when the integral value is positive.
 7. The exhaustgas purification system for an internal combustion engine according toclaim 1, wherein the basic variable calculating unit calculates thebasic variable by using an equation of a preset transfer functionmodeling temperature rising characteristics of the exhaust gas in theDOC in use of a deviation of a measured DOC inlet temperature from thetarget DPF inlet temperature, and a control gain calculated based on theexhaust gas flow rate.
 8. The exhaust gas purification system for aninternal combustion engine according to claim 1, wherein the manipulatedvariable of the temperature raising unit represents an amount of latepost-injection that is performed in a period after a main injection andthat does not directly contribute to combustion, after activation of theDOC.
 9. The exhaust gas purification system for an internal combustionengine according to claim 4, wherein determination is made that therehas been a sudden drop in the exhaust gas flow rate when the exhaust gasflow rate remains less than the threshold for more than a certain periodof time.
 10. The exhaust gas purification system for an internalcombustion engine according to claim 2, wherein the manipulated variableof the temperature raising unit represents an amount of latepost-injection that is performed in a period after a main injection andthat does not directly contribute to combustion, after activation of theDOC.
 11. The exhaust gas purification system for an internal combustionengine according to claim 3, wherein the manipulated variable of thetemperature raising unit represents an amount of late post-injectionthat is performed in a period after a main injection and that does notdirectly contribute to combustion, after activation of the DOC.
 12. Theexhaust gas purification system for an internal combustion engineaccording to claim 4, wherein the manipulated variable of thetemperature raising unit represents an amount of late post-injectionthat is performed in a period after a main injection and that does notdirectly contribute to combustion, after activation of the DOC.
 13. Theexhaust gas purification system for an internal combustion engineaccording to claim 5, wherein the manipulated variable of thetemperature raising unit represents an amount of late post-injectionthat is performed in a period after a main injection and that does notdirectly contribute to combustion, after activation of the DOC.
 14. Theexhaust gas purification system for an internal combustion engineaccording to claim 6, wherein the manipulated variable of thetemperature raising unit represents an amount of late post-injectionthat is performed in a period after a main injection and that does notdirectly contribute to combustion, after activation of the DOC.
 15. Theexhaust gas purification system for an internal combustion engineaccording to claim 7, wherein the manipulated variable of thetemperature raising unit represents an amount of late post-injectionthat is performed in a period after a main injection and that does notdirectly contribute to combustion, after activation of the DOC.
 16. Theexhaust gas purification system for an internal combustion engineaccording to claim 9, wherein the manipulated variable of thetemperature raising unit represents an amount of late post-injectionthat is performed in a period after a main injection and that does notdirectly contribute to combustion, after activation of the DOC.